Conventional thermal, flow measuring devices usually use two temperature sensors which are embodied as equally as possible, and which are arranged in (most often pin-shaped) metal shell housings—so-called stingers—and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. For industrial application, the two temperature sensors are usually installed in a measuring tube; the temperature sensors can, however, also be installed directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. As the heating unit, either an additional resistance heating is provided, or the temperature sensor itself is a resistance element—e.g. an RTD (Resistance Temperature Device) sensor—which is heated through conversion of electrical power, e.g. through a corresponding variation in the electrical measuring current. The second temperature sensor is a so-called passive temperature sensor; it measures the temperature of the medium.
In a thermal, flow measuring device, the heatable temperature sensor is usually heated in such a way, that a fixed temperature difference arises between the two temperature sensors. Alternatively, it is also known to supply a constant heating power via a control unit.
If there is no flow in the measuring tube, an amount of heat which is constant in time is then required for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is in movement, the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat from the heated temperature sensor is transported away by the flowing medium. In order to then maintain the fixed temperature difference between the two temperature sensors in the case of a flowing medium, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow, e.g. mass flow rate, of the medium through the pipeline.
If, in contrast, a constant heating power is fed in, the temperature difference existing between the two temperature sensors as a result of the flow of the medium is lessened. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline or through the measuring tube.
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline or through a measuring tube. The dependence of the so-called heat transfer coefficient on the mass flow of the medium through the measuring tube or through the pipeline is utilized in thermal, flow measuring devices for determining the mass flow. Devices which operate according to this principle are available from the assignee under the names “t-switch”, “t-trend” and “t-mass”.
Until now, mainly RTD-elements with helically wound, platinum wires were applied in thermal, flow measuring devices. In the case of thin-film, resistance thermometers (TFRTDs), a meander-shaped, platinum layer is conventionally vapor-deposited onto a substrate. Over this is applied a further, glass layer, for protecting the platinum layer. The cross section of thin-film, resistance thermometers is rectangular, in contrast to RTD elements, which have a round cross section. The heat transfer into the resistance element and/or from the resistance element accordingly occurs via two oppositely lying surfaces, which together make up a large part of the total surface of a thin-film, resistance thermometer.
EP 0 024 327 and U.S. Pat. No. 4,083,244 show different embodiments of thermal, flow measuring devices. These can also ascertain flow direction. In this regard, flow-conditioning bodies are arranged in the flow in front of a temperature sensor.